The present exemplary embodiments relate to nanoscale discotic liquid crystalline porphyrins. In certain embodiments, they find use as high-efficiency photovoltaic materials, organic semiconducting materials, organic light emitting materials and in solar cell device implementation. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications.
In addressing the growing global energy needs, harvesting energy directly from sunlight using photovoltaic technology is a key solution that can provide renewable resources while minimizing detrimental effects on the environment. Silicon based solid state photovoltaic technology is inherently expensive and difficult for mass production, while organic photovoltaic cells have the potential to be produced inexpensively. However, organic semiconductors suffer from other drawbacks, including the scattering of electron/exciton between crystal grain boundaries.
At the present time, the large majority of photovoltaic solar cells are using inorganic semiconductors of doped silicon. The problem with this material is the expensive manufacturing processing, which is essential for good power conversion efficiency (PCE). As an alternate, conjugated polymers have already been studied for photovoltaic applications. The results are promising in terms of ease of processing large films. However, the low exciton diffusion lengths (le≈10 nm) and the low charges carriers mobilities (μ≈10−3-10−4 cm2/Vs) observed limit the PCE. Another type of semiconductors, showing better mobility values and le (≈1 μm) is the columnar discotic liquid crystals.
A power conversion efficiency of up to about 2% was reported for a self-organized liquid crystal organic solar cell by Schmidt-Mende et al. (Schmidt-Mende, et al., Science, 293, 1119-22 (2001)), who used bilayer of liquid crystalline hexaphenyl-substituted hexabenzocorone (HBCPhC12) as an electron donor and a perylene dicarboxylic acid diimide derivative as an electron acceptor in the active layer of the cell. The HBC-PhC12 has a disc-like structure and forms in room temperature liquid crystalline phase, a discotic liquid, where the molecules self-organize into a columnar structure. This structure forms, because the flat shape of the molecules allows the molecular e-orbital sticking out of the plane of each molecule to form a firm bond between molecules in adjacent layer in the same fashion as in the graphite structure.
Liquid crystals, especially two dimensionally ordered discotic columnar liquid crystals, can form a long range one dimensional self-assembled architecture and can be formed into large area monodomain relatively easily using well established alignment technology developed in the liquid crystal display industry. The present invention relating to the design and synthesis of discotic columnar liquid crystalline porphoryin derivatives is an extension of this idea.
As will be described in detail, the present embodiments provide a broad temperature range discotic columnar phase capable of having a pronounced photovoltage. The materials are capable of being used as photovoltaic materials, organic semiconductors and organic light emitting materials. Although the drawings, discussions and descriptions are mainly directed toward the preparation of the said materials, photovoltaic devices and methods, it is to be understood that the principles of the present invention are applicable to any type of devices that uses a discotic liquid crystal as a photoresponsive semiconductor (For example, photo sensors, electrophotographic receptors, diodes, transistors, memory arrays and the like).